Basicity of Amines

the basicity of an amine is related to the availability of the lone pair of electrons on the N atom

Kb is the measure of basicity of an amine and can be quoted as pKb

a strong base has a large Kb value and a small pKb value

the inductive effect - alkyl groups push electrons in the C-N bond towards the N increasing the electron density on the N atom this means the electrons are more available to donate so the base is stronger

this means methylamine is stronger than ammonia and dimethylamine is stronger than methylamine

phenylamine is a weak base due to the ring of stabilising delocalised electrons and because the lone pair is not fully available to donate

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More characteristics

Solubility of Amines:

primary and secondary amines are soluble in water and form hydrogen bonds with it

tertiary amines are insoluble as there are no H atoms bonded to the N so no hydrogen bonds can be formed with the water

the higher the Mr of the amine the lower the solubility

Amines with Acids:

amines dissolve readily in acids making an alkyl ammonium salt

R-NH2 + H+ --> R-NH3+

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Preparation of Amines

1) A haloalkane with ammonia:

R-Hal + 2NH3 --> R-NH2 + NH4Hal

this method is rarely used as a mixture of primary, secondary and tertiary amines and quaternary ammonium salts is formed

have to use an excess of ammonia﻿

2) Reduction of Nitriles:

R-CN + 2H2 --> R-CH2NH2

catalytic hydrogenation using either H2 with Ni catalyst or NaBH4

3) Reduction of a nitro group (forms aromatic amines):

e.g. C6H5NO2 + 3H2 --> C6H5NH2 + 2H2O

in industry it's carried out in the presence of an Ni catalyst

in the lab the H2 is obtained from the reaction HCl with Sn or Fe

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Reactions of Amines

Reaction with haloalkanes:

the mechanism is nucleophilic substitution

R-NH2 + R1-Hal --> RNHR1 + H-Hal

in excess of the the haloalkane further substitution takes place forming a tertiary amine from the secondary amine

if yet further substitution happens a quaternary ammonium salt is formed

no further substitution happens after this as the quaternary ammonium salt is unable to act as a nucleophile as there is no lone pair of electrons on the N

Reaction with acyl chlorides:

amines react with acyl chlorides to form substituted amides

the mechanism is nucleophilic addition-elimination

R-NH2 + R-COCl --> R-CONHR + HCl

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Uses of Amines and Quaternary Ammonium Salts

There are many uses of amines and quaternary ammonium salts:

amines can be used to make polymers such as polyurathane (lycra) and polyamides (used in carpets)

aromatic amines are used in the production of synthetic dyes

quaternary ammonium salts are used as cationic surfactants which are used in things such as fabric and hair conditioners